Advances in medical imaging have largely paralleled the growth of digital technology. Through digital technology, it is now feasible to gather, store, analyze and search large amounts of information at a rapid pace. Enhanced imaging and image post-processing have also been made possible by advances in digital technology, and are now commonly used in almost all modalities of medical imaging. Broadly, medical imaging incorporates radiology, nuclear medicine, endoscopy, investigative radiological sciences, thermography, medical photography and microscopy.
Radiology is the branch of medical science dealing with medical imaging for the purpose of diagnosis and treatment. The practice of radiology often involves the usage of X-ray machines or other radiation devices to perform the diagnosis or administer the treatment. Other practices of radiology employ techniques that do not involve radiation, such as magnetic resonance imaging (MRI) and ultrasound. As a medical field, radiology can refer to two sub-fields, diagnostic radiology and therapeutic radiology.
Diagnostic radiology is concerned with the use of various imaging modalities to aid in the diagnosis of disease or condition. Therapeutic radiology or radiation oncology uses radiation to treat diseases such as cancer through the application of radiation to targeted areas.
Newer technology and advanced techniques allow for improved image collection with the application of computerized tomography (CT) to medical imaging techniques. Conventional medical imaging processes involving CT scans will produce a series of 2-dimensional images of a target area which are subsequently combined using computerized algorithms to generate a 3-dimensional image of the target area. While CT can generate much more detailed images of soft tissues, it also exposes the target area (and thus, a patient) to more ionizing radiation, which may be potentially harmful or otherwise undesirable.
A typical configuration for a radiology device includes a radiation source for emitting the radiation (e.g., X-rays) used for imaging and one or more imaging devices corresponding to a radiation source for receiving incoming radiation after passing through the target volume. The beams collected by the imagers are subsequently used to generate a display (i.e., one or more images) of the targeted volume.
Generally, the imager used for X-ray is usually of the scale of the size of the object being imaged. These imagers often comprise integrated circuits in the form of amorphous-silicon (a-Si) thin film transistor (TFT) arrays. Among various technologies that have been developed based on amorphous silicon TFT arrays, the most widely used ones are “indirect detectors.” These detectors are based on amorphous silicon TFT/photodiode arrays coupled to radiation scintillators (e.g., X-ray scintillators). Scintillators are detectors of radiation through their inherent capability of converting incident radiation into photons. The amount of conversion of X-ray radiation photon to light photon is measured as the efficiency of the scintillator. The efficiency of the scintillator is indicative of the amount of radiation the patient has to be exposed to get a certain quality of the image. If the efficiency is higher, the same quality of image can be obtained with relatively less radiation exposure to the patient.
Radiation dosages determine the amount of radiation exposure to the patient. Generally, diagnostic imaging involves less dosage than therapeutic imaging. Also the radiation applied is classified in terms of energy. Kilo-voltage (KV) imaging involves less energy than (mega-volt) MV imaging. The efficiency of the scintillator is also related to the amount of energy in the radiation, in other words whether it is a KV radiation or MV radiation. For example, a typical X-ray tube can generate 10 to 150 KV (KV) of energy. In radio-therapy, electrons/gamma radiation devices can produce about 1-100 MV (MV) of energy. Since MV radiation involves higher energy radiation photons, the probability of interaction with matter is lower.
Several solutions have been proposed to increase the efficiency of the scintillators used in MV imaging. These include making the scintillator wider or thicker or designing a segmented scintillator. Unfortunately, thick and segmented scintillators are very expensive to build and can suffer from performance problems. For example, increasing the thickness of the scintillator also leads to degraded light performance due to degradation of signal to noise ratio (SNR). Thicker scintillators also suffer degradation in resolution due to beam divergence. Beam Divergence is an angular measure which results in an increase in beam diameter which correlates with increasing distance from the optical aperture of emergence. Due to its overall thickness, the collection and distribution of photons which should represent the image suffers as they tend to diverge. Segmented scintillators also suffer from lower resolutions and are limited for usage at one specific source-imager distance—thereby resulting in limited image data acquisition distances and methods.